The present invention relates to communication networks, and particularly, the present invention relates to providing guaranteed quality of service in a packet switched network.
In communications technology, there is an ever-increasing demand for high-performance networks, and in particular, a demand for high-performance Internet access. This increased demand has led to the development of improved networks capable of handling larger volumes of data with smaller delays. Nonetheless, these improved networks each have their own shortcomings.
Communications networks like the Internet are generally formed with a number of transmission links interconnected with switches. A transmission link is any medium through which signals are communicated and can be single or multiple twisted pairs, optical fiber, coaxial cable, radio links, or other mediums. A switch is a device with one or more input ports and one or more output ports. The switch directs bits arriving at an input port to the appropriate output port. Switching in communications is accomplished using one of two methods: circuit switching and packet switching.
Circuit switching has been used since the birth of the modern telephone system. Circuit switching entails establishing an end-to-end connection between two points before any information is communicated. A telephone connection is one example of a circuit-switched connection. In early circuit-switched networks, information could only be communicated over the links if the links were not being used by another party and if an end-to-end connection was already established.
To make the circuit-switched network more economical, Time Division Multiplexing (TDM) was developed. TDM is a method that enables multiple end-to-end connections to share the same link. More specifically, using TDM the respective data streams are transmitted for a designated time period in a round robin fashion over a shared transmission link. For instance, referring to the illustration of FIG. 1, to multiplex N data streams using TDM, the first stream is transmitted for T(1) seconds, then the second stream for T(2) seconds, and so on until the N-th stream is transmitted for T(N) seconds. The cycle then repeats starting again with the first stream. Hence, each data stream gets a fraction of the total transmission rate for the shared link, i.e., the stream i (i=1, . . . , N) gets a fraction       T    ⁡          (      i      )            [                  T        ⁡                  (          1          )                    +              T        ⁡                  (          2          )                    +      …      +              T        ⁡                  (          N          )                      ]  
of the transmission rate of the link. So, using TDM, a link with transmission rate R is capable of carrying data streams with rate R(1), . . . , R(N) provided that R(1)+ . . . +R(N) is less than R. Therefore, a circuit-switched network with TDM provides a guaranteed bandwidth to a connection from the source of the connection to its destination.
Despite the ability to send multiple data streams through a shared link at a guaranteed rate for each data stream, when a particular data stream does not need its time slot on the link, that fraction of the link capacity is wasted because other data streams are not permitted to use that time slot. Thus, although the circuit-switched network offers the user guaranteed bandwidth available at any time, when the user is not utilizing its allocated bandwidth, the link lays idle for the user""s time slotxe2x80x94unavailable for use by any other party, leading to an inefficient utilization of available resources.
Even though there are inefficiencies inherent in circuit switching, such switching has proven satisfactory to carry voice data. Non-voice data, however, tends to be more xe2x80x9cburstyxe2x80x9d in nature. Hence, when a data burst arrives, having to wait through TDM cycles for transmission will make the transmission of data appear slowxe2x80x94an appearance that might be avoided if all resources were available for use.
To avoid the inefficiencies of circuit switching, the communications industry has made a shift to xe2x80x9cpacket switchingxe2x80x9d for non-voice type data transmission. Packet switching does not establish a single end-to-end connection for each data stream. Rather, the data stream is divided into xe2x80x9cpackets,xe2x80x9d each of which carries data as well as various identifying information about each packet. Such identifying information includes routing information, for instance, source and destination addresses for the packet. When a packet arrives at a switch, it is stored in a buffer. The switch looks at the routing information in the packet and routes the packet to the appropriate output port. The packets are then forwarded one xe2x80x9chopxe2x80x9d (or switch) at a time, until they reach their destination. Each packet in a particular data stream does not necessarily follow the same path to its destination as the others.
In most networking protocols, the networking system is hierarchically divided into subnetworks that are interconnected. The nodes (e.g., personal computers) in each subnetwork are addressed by a xe2x80x9cLayer 2xe2x80x9d address (sometimes called a MAC address). A subnetwork delivers packets between any two of its nodes. These packets are labeled with the Layer 2 addresses of the source and destination nodes of the subnetwork.
Subnetworks can also be interconnected to one another with the use of specialized switches. These switches determine how to handle a packet based either on the Layer 2 addresses in the packet or on some other routing information. For instance, a switch interconnects a number of subnetworks through its several ports. For each of its ports, the switch maintains a list of Layer 2 addresses of the computers of the subnetwork(s) that is(are) attached to the port. When the switch receives a packet, the switch checks the port lists and identifies the port that is connected to the Layer 2 address for the destination of the packet. The switch then transmits the packet on that identified port. Other networking protocols additionally add special routing information in each packet. That routing information may be a connection number or a pair of geographically arranged addresses, as in the case of the Internet Protocol.
Since packets from different data streams are all routed over the same transmission lines, a scheduling method by which to place the packets from the different streams onto the transmission lines is required. One such scheduling method is statistical multiplexing (SM). In contrast with TDM, SM does not allocate fixed periodic fractions of the transmission rate of the link to the different data streams. Instead, the SM schedules packets for transmission depending on the actual needs of the different data streams. In its simplest form, illustrated in FIG. 2, statistical multiplexing schedules packets on the transmission line in the order of their arrival at the switch, i.e., on a first come, first served basis. Although such a scheduling method does not waste communication resources, it cannot guarantee a minimum transmission rate for any particular data stream. The packets simply get to their destination when they get there.
A variation of SM called xe2x80x9cdeficit round robinxe2x80x9d (DRR) can provide better transmission rate guarantees and is illustrated in FIG. 3. Under DRR, to multiplex N data streams, N integers W(1), . . . W(N), are first chosen as a weight (W) for each data stream. Then for the first data stream (DS1) up to W(1) bits are transmitted. If all of the packets of DS1 are transmitted before transmitting W(1) bits or if bit W(1) is the last bit of a packet, then a second integer D(1) is set to 0. But if bit W(1) is not the last bit of a packet, then additional bits are sent until the transmission of the packet is completed, and D(1) is set to equal the number of additional bits sent. The procedure is repeated for data streams DS2, . . . , DSN with the corresponding numbers W(2), D(2), . . . W(N), D(N). The cycle then repeats starting with DS1, but W(1) is replaced with W(1)xe2x88x92D(1). Hence, W(1)xe2x88x92D(1) bits are transmitted instead of W(1) (in other words, D(1) represents a xe2x80x9cdeficitxe2x80x9d of DS1). If there are additional bits remaining in a packet, then those Dxe2x80x2(1) additional bits are sent. The procedure is again repeated for DS2, . . . , DSN in a similar manner. When the procedure returns again to DS1, W(1)xe2x88x92Dxe2x80x2(1) will replace W(1).
Therefore, using DRR, a minimum transmission rate for a data stream DSi (where i=1, . . . , N) will generally be the fraction       W    ⁡          (      i      )            [                  W        ⁡                  (          1          )                    +              W        ⁡                  (          2          )                    +      …      +              W        ⁡                  (          N          )                      ]  
of the transmission rate for the link. Moreover, if some data streams do not need their guaranteed allocation, then the other connections can utilize the unused resources and will do so automatically. Many variations of DRR are known in the art, including Weighted Fair Queuing (WFQ), Generalized Processor Sharing (GPS), and Class Based Queuing (CBQ).
Despite DRR""s ability to generally provide some minimum transmission rate, the capacity for the transmission of data at a particular rate is still not reserved like it is in circuit-switched networks. Hence, there is still the possibility that delivery of a message will be delayed. Therefore, most data communication service providers, although they utilize statistical multiplexing for packet transmission, will not guarantee any minimum quality of service. xe2x80x9cQuality of servicexe2x80x9d refers herein to a minimum bandwidth provided between two points, A and B.
The advent of Asynchronous Transfer Mode (ATM) seems to combine the benefits of statistical multiplexing with the guaranteed quality of service of circuit switching. While ATM sends data in packets, or cells, ATM sets up a xe2x80x9cvirtual channelxe2x80x9d before transmitting any data. All of the frames follow the same route along the virtual channel, thereby guaranteeing the order of cell arrival. Further, because data is sent in cells, these cells can be statistically multiplexed, and can use all available resources efficiently while guaranteeing a minimum quality of service. To allow all incoming data streams access to a shared link, ATM monitors the number of packets or cells received from a particular data stream. If the number received during a time interval exceeds a threshold designated for the incoming data stream, then packets/cells are discarded.
Nonetheless, ATM is a complicated protocol and expensive to implement, requiring complex hardware and network management software. Further, in addition to setting up and maintaining virtual channels, ATM requires the reformatting of received data packets into special ATM cells that include a virtual channel identifier and a virtual path identifier unique to ATM. So while ATM has been used by telephone service providers, because of its expense and complexity it is not generally used for Internet or other data communications.
Therefore, a simplified system and method for providing a guaranteed quality of service for Internet and other packet-switched data communications is desirable.
In order to overcome the deficiencies in communication networks described above, a system and method in accordance with the invention provides a simple, yet flexible packet-switching system that can guarantee quality of service between two points. In particular, such a system includes a plurality of linked nodes. Packet Switches, such as standard routers, are coupled to the nodes.
Service Level Agreements, or SLA""s, are defined between pairs of Packet Switches and guarantee a minimum quality-of-service (minimum bandwidth) between the two packet switches. When a packet arrives at a node from a packet switch, the node inspects certain classification information contained within the packet. In one embodiment, such classification information is the source and destination identifiers (e.g., addresses) of the packet, while in other embodiments classification information additionally includes other information. Using the classification information, the packet classifies the packet with an SLA. A scheduler in the node ensures that packets from each SLA are scheduled for transmission at at least the minimum data rate corresponding to the SLA.
In one embodiment, each SLA has its own queue in the node. In such an embodiment, a scheduler schedules the packets for transmission using a statistical multiplexing method. In one embodiment, such a method is a DRR method. In another embodiment, such a method is a modified DRR method referred to herein as a xe2x80x9cDeficit Golden Ratioxe2x80x9d (DGR) method. A DGR method in accordance with the invention, redistributes the order of the packets that would otherwise be output using a DRR method, thereby eliminating jitter and delay often caused by DRR scheduling.
In a second embodiment, the SLA""s served by a node share a queue. In other words, all packets, no matter which SLA they are classified with, are placed in the same queue. However, a statistical multiplexing scheduling method referred to herein as SLA Early Discard is utilized to ensure the minimum bandwidth of all SLAs. If the number of packets in the queue for a particular SLA have reached or exceeded a threshold value, then no additional packets for that particular SLA are added to the queue. Any newly arriving packets for the SLA are discarded. If the packets for the particular SLA have not reached the threshold, then additional packets for that SLA can be added to the queue.
In addition to classification and scheduling, nodes in accordance with the invention further implement congestion control. When packets for a particular SLA are transmitted at a rate higher than the minimum rate designated for the particular SLA, then downstream nodes receiving those packets may become congested. In one embodiment, those downstream nodes provide messages to the upstream nodes indicating that the rate of transmission for packets from the particular SLA should be reduced. In some embodiments, the node may turn off service for the particular SLA while it is congested. However, service to the other SLA""s remains turned on in the node.
Thus, a system and method in accordance with the invention can provide service to users akin to leased lines in a circuit switched system, although the system and method described herein is for packet switching. Moreover, such service can be provided without reformatting data packets as is done in ATM. In one embodiment, the nodes in accordance with the invention have ethernet interfaces and accept and forward ethernet packets without change. Hence, standard packet switches xe2x80x9cseexe2x80x9d a network in accordance with the invention as an ethernet-type network. Finally, such guaranteed quality-of-service can be provided with minimal additional hardware and software unlike other conventional networking methods such as ATM.